1. Field of the Invention
The present invention relates to a paper supply device for supplying paper one sheet at a time from a stack of multiple sheets to the processing unit of an apparatus such as a data reader or printer.
2. Description of Related Art
Roller-type paper supply devices are commonly used in printers, facsimile machines, copiers, and other office machines.
These roller-type paper supply devices typically have a cylindrical supply roller at the end in the paper feed direction, and a pressure means having a spring or other elastic means for pressing the paper to the supply roller so that the supply roller nips the paper near the leading edge of the sheet and feeds the paper into the paper path. A consistent paper supply operation can thus be achieved regardless of the stiffness (rigidity) of the paper by nipping and feeding the paper from the leading end.
Some paper supply devices also have a separation pad disposed downstream from the supply roller to separate the sheets one at a time from the stack using friction when the leading ends of the sheets slip. In this arrangement, locating the separation pad and supply roller close together affords better separation of multiple sheets.
However, if the supply roller is located at the end in the feed direction and the ends of the sheets are not aligned flush with each other, the end of the topmost sheet may not reach the point where the paper contacts the roller. The supply roller may therefore not be able to nip the end of the sheet, and it may not be possible to convey the sheet into the paper supply path.
Some means of aligning the ends of the sheets is therefore typically provided in such paper supply devices. The methods used to align the sheets are generally one of the following types.
In machines such as laser printers and photocopiers, the paper is generally stocked horizontally in a flat stack in a paper cassette that reliably positions the sides and top and bottom of the stack. See, for example, Japanese Patent Laid-Open Publication (kokai) H8-277044.
In inkjet printers, however, the paper is typically placed standing up vertically with the leading ends of the pages at the bottom so that gravity is used to drop the leading edge of the sheets to a pad at the bottom. See, for example, Japanese Patent Laid-Open Publication (kokai) S62-153033.
This method of horizontally stacking the paper to precisely position the sides and top and bottom surfaces requires a paper cassette, however, which makes it difficult to make the paper supply device smaller.
Setting the paper in a vertical stack and using gravity to drop the leading edge of the paper to the bottom pad makes it possible to reduce the size of the paper supply device. However, because the paper cannot be supplied if it is not stacked vertically, this limits the orientation in which the printer and paper supply device can be used.
Conventional roller-type paper supply devices thus use the transportation force created by a spring urging the supply roller against the paper to take the single sheet closest to the supply roller from the stack of multiple sheets loaded in the paper supply device. The problem with this arrangement is that if the spring is too strong, two or more sheets are supplied together, but if the spring is too weak, paper supply may be unreliable or impossible. Determining the appropriate spring force (transportation force) is therefore difficult.
An object of the present invention is therefore to solve these problems and provide a paper supply device that is small, simply constructed, and can be used in various postures while still reliably supplying paper one sheet at a time from a stack of multiple sheets even when the edges of the sheets are somewhat misaligned.